Lobe weight gain and vascular, alveolar, and peribronchial interstitial fluid pressures.

Interstitial fluid movements in acute pulmonary edema were studied by recording interstitial fluid pressure [Px (f)] relative to pleural pressure (atmospheric), together with lobe weight gain or loss (delta W). Px (f) was measured by wicks inserted between lobar bronchus and artery while alveolar pressure (PA) was fixed at either 5 or 20 cmH2O. When vascular pressure (Pvas) was raised abruptly from -5 to +25 cmH2O by air inflation for 60 min, Px (f) became abruptly less negative, then remained stable. However, during vascular inflation with plasma, delta W began a steady increase, but plotted against delta W, Px(f) became less negative in several phases. After an immediate rise due to interdependence effects following vascular distension, Px (f) remained almost unchanged for 4-7 min as delta W increased 15-80% of initial lobe weight (Wi), representing a transport lag between leakage and measuring sites and suggesting that interstitial edema was not homogeneous. Next, Px (f) increased progressively as weight increased a further 70-200% of Wi and finally slowed its rise near zero pressure. When Pvas was lowered, Px (f) became abruptly more negative, again by interdependence; however, as delta W then decreased 20-50% of Wi over 30 min, Px (f) did not change consistently. It was possible to relate the rate of weight gain occurring between 2 and 5 min after Pvas was raised to two pressure gradients, Pvas - Px (f) and Pvas - PA, and to relate the time lag to filtration rate and Pvas - Px (f).